Image degradation due to cardiac and respiratory motion remains a challenge for cardiac PET-MR imaging. Here we propose a simultaneous dual-phase PET and Coronary MR angiography (CMRA) acquisition and reconstruction framework that allows for the visualisation of coronary anatomy, estimation of ventricular function and motion-corrected myocardial PET in a single efficient examination. A validation study in healthy subjects shows that left ventricular ejection fraction can be estimated from dual-phase CMRA images at 3T. Results from patients with cardiovascular disease show improvements in respiratory motion corrected dual-phase CMRA and in PET image quality when applying both cardiac and respiratory motion correction.
An ECG-triggered free-breathing dual-phase CMRA sequence is acquired simultaneously with list-mode cardiac PET data (Fig1a). CMRA data is acquired with a 3D T1-weighted spoiled gradient echo sequence, with a fully sampled golden-step Cartesian with spiral profile order sampling trajectory4. 2D image navigators5 are acquired before the CMRA data in each heartbeat. Non-rigid respiratory and cardiac motion is estimated from MR images, and the MR-derived deformation fields are used to correct for both the CMRA (respiratory motion correction for each cardiac phase6) and the simultaneously acquired PET data (respiratory and cardiac motion correction) (Fig1b).
Six healthy subjects were scanned on a Biograph mMR 3T scanner (Siemens Healthcare, Germany) using a prototype implementation of the proposed gradient echo dual-phase CMRA sequence (resolution = 1x1x2mm, FOV = 304x304x80-88mm, coronal orientation, TR/TE = 3.7/1.7ms, FA=12°, T2prep duration = 50ms). Two subject-specific trigger delays were defined coinciding with the systolic and diastolic rest periods and an acquisition window ranging from 89 to 104ms was used. Additionally, a conventional multi-slice and multi-breath-hold 2D short-axis cine acquisition was performed to validate the estimation of left ventricular function. The whole PET-CMRA framework was tested in three patients with documented ischemic heart disease, using the same acquisition parameters described for the healthy subjects, with 18F-FDG list-mode PET data acquired simultaneously during the whole dual-phase CMRA acquisition. In patients acquisition was performed ~1-2 min after injection of a Gd-based contrast agent.
Dual-phase CMRA and PET data were reconstructed without motion correction (NMC) and with the proposed motion correction scheme (MC). PET images were also reconstructed with respiratory motion correction only (RespMC) for comparison purposes. PET image reconstruction was performed offline using a reconstruct-transform-average7 motion correction in Siemens e7 Tools (OSEM, 3 iterations, 21 subsets, PSF modelling, 2.03x2.08x2.08mm voxel size, 127x344x344 matrix size).
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